Polarizing plates are widely used in displays such as liquid crystal displays and, particularly in recent years, in a variety of mobile devices such as smart phones and slate PCs. For use in mobile devices, polarizing plates are increasingly required to be thin and light. On the other hand, polarizing plates are also required to have high heat resistance.
However, conventional polarizing plates can cause a problem called “red discoloration”, in which light in the red region leaks from polarizing plates having undergone a heat resistance test (generally at 80 to 85° C. for 500 to 750 hours).
Various methods have been proposed to suppress discoloration such as red discoloration.
Conventional methods for suppressing discoloration or decoloration of polarizers include, for example, a method of adding sulfate ions to polarizers [Japanese Patent Laying-Open No. 2009-193047 (PTD 1)]; a method in which a process of immersing a film in an aqueous solution with a pH adjusted in a specific region is added to a polarizer producing process [Japanese Patent Laying-Open No. 2009-282137 (PTD 2) and Japanese Patent Laying-Open No. 2008-070571 (PTD 3)]; a method of forming a specific protective layer on a polarizer [Japanese Patent Laying-Open No. 2008-065222 (PTD 4)]; a method of controlling the ratio between the contents of iodine and potassium in a polarizer [Japanese Patent Laying-Open No. 2007-114581 (PTD 5)]; a method of improving the orientation of I3− adsorbed in a polarizer [Japanese Patent Laying-Open No. 2004-341503 (PTD 6)]; and a method of adding zinc ions to a polarizer (Japanese Patent Publication No. S60-033245 (PTD 7), Japanese Patent Publication No. H02-034001 (PTD 8), and Japanese Patent Laying-Open No. 2000-035512 (PTD 9)].